The present invention relates to a head actuator mechanism which moves a magnetic head for recording and reading information with respect to a magnetic disk so that the magnetic head is positioned on a desired position, and to a magnetic disk drive including the head actuator mechanism.
A magnetic disk drive, for example, a small-sized hard disk drive (hereinafter, referred to simply as HDD) has been conventionally used as a built-in memory of a lap-top type or notebook type personal computer.
In general, in the HDD, a positioning control for the magnetic heads is carried out by means of a servo system which executes speed control and position control largely classified. The servo system positions the magnetic heads at a target position by controlling a head actuator mechanism for supporting and moving the magnetic heads in the radial direction of a magnetic disk.
In recent years, with high performance of a personal computer, there has been deeply made a request of an HDD which is made into a small size and has a large memory capacity. However, in order to increase the memory capacity of the HDD, a track density and linear recording density of the magnetic disk serving as a recording medium need to be improved so that high recording density of the magnetic disk can be achieved.
In order to realize the aforesaid high recording density, various trials have been made. In accordance with increase in the recording density, there is required a technique of positioning the magnetic head to a target track on the magnetic disk with high precision. For this reason, the head actuator mechanism has been variously devised together with control technology of the aforesaid serve system.
Generally, the HDD is provided with a case made of an aluminum alloy or the like, in which are arranged a magnetic disk, a spindle motor for supporting and rotating the magnetic disk, a rotary type head actuator mechanism for rotatably supporting the magnetic head with respect to the magnetic disk, a voice coil motor (hereinafter, referred simply to as VCM) for driving the head actuator mechanism, and a circuit board. The magnetic disk makes a high rotational speed motion by means of the spindle motor. Also, the circuit board is mounted with various circuit components such as a magnetic head amplifier for amplifying a read signal from the magnetic head.
The head actuator mechanism includes a suspension for supporting a magnetic head, a support arm for supporting the suspension and transmitting a driving force, and an actuator body which supports the support arm and is driven around a rotational shaft by a driving force of the VCM.
The VCM includes a driving coil mounted on a substantially V-shaped coil holding frame, a permanent magnet, and yokes. The coil holding frame extends from the actuator body in a direction reverse to the support arm. The coil holding frame and the driving coil are rotatable integrally with the head actuator mechanism. Further, the permanent magnet and the yoke constituting a magnetic circuit are fixed on the case side, and are arranged facing each other on opposite sides of the driving coil. The VCM drives the actuator by an interaction of a magnetic field generated from the permanent magnet with an electromagnetic force generated by exciting the driving coil.
The servo system controls a driving current of the VCM on the basis of a servo data previously recorded in a servo area of the magnetic disk, and then, carries out a drive control of the actuator.
In the HDD having the construction as described above, the actuator is driven with high precision so that the magnetic head is high precisely positioned to a desired position; however, there is a hindrance when positioning the magnetic head. The hindrance is a mechanical vibration generated in components such as the support arm constituting the actuator, or the like.
The mechanical vibration of the head actuator mechanism includes a resonance peak due to a rotary spring characteristic of ball bearings provided in the actuator body, and peaks resulting from resonance modes of the head actuator mechanism itself.
The major resonance mode of the head actuator mechanism is estimated to be generated by a mode wherein the driving coil of the VCM and the coil holding frame holding the driving coil are deformed in a rotating (circumferential) direction of the magnetic disk when driving the head actuator mechanism. Further, the major resonance mode resulting from the structure of the head actuator mechanism itself is a high frequency of 1 kHz or more, and affects the servo system for carrying out a positioning control of the magnetic head. This is a factor of malfunction such as an off-track error.
More specifically, the vibration of the head actuator mechanism is a factor of lowering a positioning accuracy of the magnetic head, and in particular, causes a problem of lowering a recording density (track density) in a track direction.
Therefore, in order to prevent a bad influence to the servo system as much as possible, there are required to eliminate the mechanical vibration of the head actuator mechanism as much as possible and to provide a design or device for increasing a resonance frequency of the support arm or the like.
Further, there is a need of decreasing an off-track resulting from vibration applied from the outside of the HDD, vibration generated from the spindle motor and others in the HDD and the like. In this case, a gain cross frequency, at which gain crosses 0 dB and which is one of the open loop characteristic of the servo system, must be increased as much as possible.
A method of increasing the gain cross frequency in the servo system has been disclosed in Jpn. Pat Appln. KOKAI publication No. 51-36924, or in a document "Track follow-up control of magnetic drive 2-stage access servo system" (VOl. J75, No. 11, pages 653 to 662) published by Institute of electronic information and communication, for example.
In the aforesaid Publication and document, there has been proposed a system which is provided with a main actuator (VCM) for integrally moving a plurality of magnetic heads with a long stroke, and an auxiliary actuator (composed of a piezo-electric element) for finely moving the magnetic heads independently from each other.
The system has been also disclosed in Jpn. Pat. Appln. KOKAI publication No. 51-39012. The system disclosed in the above Publication is constructed in the following manner. More specifically, the magnetic head follows a narrow track pitch by a double servo system so that the main actuator having a heavy weight is driven in a low frequency band and the auxiliary actuator having a light weight is driven in a high frequency band.
Further, Jpn. Pat. Appln. KOKAI publications No. 51-36924, No. 3-69072, No. 3-102684 and No. 3-183070 disclose a system which is provided with an auxiliary actuator on each support arm supporting a magnetic head. In this system, individual magnetic heads are movable independently from each other by means of the auxiliary actuators.
In Jpn. Pat. Appln. KOKAI publications No. 3-69072, No. 3-102684 and No. 3-183070, there is a description on the case where the auxiliary actuator is applied to a dedicated servo system. More specifically, there is a description on a system in which the auxiliary actuator is used as means for correcting a so-called thermal off-track.
The thermal off-track is caused by the phenomenon that a plurality of support arms individually deform in a radius direction of the magnetic disk by a change in temperature on the outside and inside of an apparatus. For this reason, there has been employed a method of correcting only the DC component of the magnetic head on the basis of a positional information from a servo surface of the magnetic disk and a positional information from sectors of each data surface. In this case, an auxiliary actuator need to be provided for respective support arms, and further, must be individually controlled.
In the HDD having the construction mentioned above, since the main actuator and the auxiliary actuator are constructed integrally, a resonance frequency of the main actuator appears as a resonance frequency in the auxiliary actuator. Thus, it is difficult to broaden a servo band with the use of the auxiliary actuator.
In the case of a conventional head actuator mechanism which is provided with the auxiliary actuator for each support arm, the following problems is caused.
First, a piezo-electric element constituting the auxiliary actuator is required only for the number corresponding to that of the support arms. For this reason, in the case where a variation exists in characteristic of each piezo-electric element, there is high possibility to generate difference in fine movement of magnetic heads, flying characteristic of the magnetic head sliders or the like.
Further, a cable and driving circuit for driving the auxiliary actuator are required for the numbers corresponding to that of the support arms, and these auxiliary actuators must be individually controlled. Thus, the control becomes complicate.
Furthermore, the auxiliary actuators for the support arms are driven independently, so that a resonance mode of the whole structure of the actuator is induced. As a result, it is difficult to increase a servo band in the magnetic head positioning system.